1. Field of the Invention
The present invention relates to methods and apparatus for the separation of a catalyst and hydrocarbon materials in a fluidized catalytic cracking (FCC) unit. More particularly, the present invention relates to an improved method and apparatus for reducing the contact time between catalyst and hydrocarbon materials in a stripping zone after "separation" in a conventional separator.
2. Discussion of the Prior Art
The field of catalytic cracking, particularly fluid catalytic cracking, has undergone significant development improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered, requiring refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in an FCC installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition with a hydrocarbon feed generally upwardly through one or more riser conversion zones (FCC cracking zones), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 1000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst.
Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion zone. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by a separator, which could be a mechanical means, and/or stripping gas in a separate catalyst stripping zone. Hydrocarbon conversion products separated and materials stripped from the catalyst are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
Of particular interest has been the development of methods and systems for separating catalyst particles from a gas suspension phase exiting the riser and containing catalyst particles and vaporous hydrocarbon product materials, particularly the separation of high activity crystalline zeolite cracking catalysts, under more efficient separating conditions so as to reduce overcracking of hydrocarbon conversion products and promote the recovery of desired products of a hydrocarbon conversion operation. Cyclonic equipment is now typically used for efficient separation of fluidizable catalyst particles from the gas suspension phase. However, present day cyclonic equipment often permits an undesirable extended residence time of the product vapor within a large reactor vessel. This extended residence time reduces the desired product yield by as much as 4% through non-selective thermal cracking. Recent developments in this art have been concerned with the rapid separation and recovery of entrained catalyst particles from the gas suspension phase.
Various processes and mechanical means have been employed heretofore to effect rapid separation of the catalyst phase from the hydrocarbon phase at the termination of the riser cracking zone, to minimize contact time of the catalyst with cracked hydrocarbons. A representative one of these is shown in FIG. 1 and discussed below by way of general background for the present invention.
FIG. 1 in the present application corresponds to a simplified illustration of FIG. 2 from Anderson et al, U.S. Pat. No. 4,043,899, where similar reference numbers have been utilized to illustrate similar structures in the two figures. Anderson et al discloses a method for rapid separation of a product suspension, comprising the vaporous hydrocarbon product phase and fluidized catalyst particles (HYC+CAT, as seen entering riser conversion zone 24), by discharging the entire suspension directly from the riser conversion zone into a cyclone separation zone 4. The cyclone is modified to include a separate cyclonic stripping of the catalyst separated from the hydrocarbons vapors in an auxiliary stripper. The cyclone separator is modified to include an additional downwardly extending section comprising a lower cyclone stage 11. In this arrangement, catalyst separated from the gasiform material in the upper stage of the cyclone, slides along a downwardly sloping helical baffle 12 to the lower cyclone, where stripping steam (STM) is introduced to further separate entrained hydrocarbon products from the catalyst recovered from the upper cyclone. The steam and stripped hydrocarbons are passed from the lower cyclone through a concentric pipe 8, where they are combined with the hydrocarbon vapors separated in the upper cyclone.
The separated, stripped catalyst is collected and passes from the cyclone separator 4 by conventional means through a dipleg 22 into a catalyst bed 60 in the bottom of reactor vessel 26 and out catalyst exit 44. This lower portion of vessel 26 also acts as a catalyst stripping section, comprising baffles 32, 34, and 36, with steam being supplied to the catalyst bed thereunder. Vaporous material separated in cyclone 4 can also be discharged into cyclone 52 and subsequently passed by way of conduit 54 into chamber 46 and withdrawn therefrom by conduit 48 for eventual fractionation.
While the Anderson et al patent, along with U.S. Pat. No. 4,219,407 to Haddad et al (herein incorporated by reference), represent improvements in the field of rapidly stripping of hydrocarbon materials from catalyst particles, there is still a need to further reduce total contact time between hydrocarbon materials and catalysts to reduce, to the extent possible, non-selective cracking. Thus, although a substantial amount of catalyst stripping occurs in catalyst bed 60, the stripped hydrocarbon material still contacts with additional catalyst particles as it is carried upward through the catalyst bed and into the entrance of cyclone 52 and from there to chamber 46 and eventual fractionation. This increased hydrocarbon material/catalyst contact contributes to uncontrolled and undesired cracking of the hydrocarbon materials.
It can be seen that at each stripper stage in FIG. 1, represented by baffles 32, 34 and 36, the hydrocarbon materials stripped from catalyst in the lower portion of vessel 26 undergo further catalyst contact while making their way to the surface of the catalyst bed. Because the catalyst bed acts as a lower seal to the dipleg 22 (and thus prevents the flow of hydrocarbon-laden gas through dipleg 22 into the catalyst bed), often the dipleg must be extended deep within the catalyst bed in order to provide a sufficient seal. This depth requirement, plus the desirability of multistage stripping (in order to ensure that a high percentage of hydrocarbon material is removed from the catalyst particles) requires a rather substantial volume of catalyst in the catalyst bed 60, which volume serves to increase the uncontrolled residence time of hydrocarbon material with catalyst particles.